The SRD only gives a means to determine costs and weights of weapons for Large and Small versions based on the Medium weapons. Any other supplied values are the author's best determination.

Loading a heavy great harquebus is a full-round action that provokes attacks of opportunity.
Normally, operating a heavy great harquebus requires two hands. However, you can shoot, but not load, a heavy great harquebus with one hand at a -4 penalty on attack rolls. You can shoot a heavy great harquebus with each hand, but you take a penalty on attack rolls as if attacking with two one-handed weapons. This penalty is cumulative with the penalty for one-handed firing.